The present invention relates to an oxygen sensor adapted for the air-fuel ratio control of, for example, a vehicular internal combustion engine, and an air-fuel ratio control apparatus of an internal combustion engine using the oxygen sensor.
In internal combustion engines of automobiles and the like, the air-fuel ratio of an air-fuel mixture supplied thereto should be controlled so as to be always in the vicinity of the stoichiometric (SM) ratio or the theoretical air-fuel ratio, in order to make the most of the engine performance. This air-fuel ratio control is very important also because a three-way catalyst, which is used to remove harmful substances in exhaust gas, serves to simultaneously remove, with high efficiency, CO, NO.sub.X, and HC from exhaust gas which is produced when an air-fuel mixture having an air-fuel ratio controlled within a very narrow range in the vicinity of the aforesaid stoichiometric value is burned.
An air-fuel ratio control apparatus is used for the air-fuel ratio control of these internal combustion engines. The control apparatus of this type serves to control the injection quantity of a fuel injection valve in accordance with an oxygen concentration detection signal delivered from an oxygen sensor, which is disposed, for example, in an exhaust passage of the engine, on the upper-course side of the three-way catalyst.
More specifically, when the air-fuel ratio of the air-fuel mixture supplied to the engine varies between the fuel-rich and -lean sides, with respect to the stoichiometric ratio, the concentration of oxygen in the exhaust gas changes, so that the output value of the oxygen sensor also changes across predetermined discrimination value V.sub.X. An electronic control unit varies the amount of fuel supply in accordance with the output value of the oxygen sensor, more specifically, the direction of the change of the sensor output value with respect to predetermined discrimination value V.sub.X, and the time elapsed after value V.sub.X is crossed by the sensor output value. Thus, the air-fuel ratio is controlled in the vicinity of the stoichiometric ratio.
In a practical oxygen sensor adapted for such air-fuel ratio control, an oxygen ion conducting solid electrolyte is held between a pair of electrodes, which are brought into contact with atmospheres containing oxygen at different concentrations, thus forming an oxygen concentration cell. The concentration of oxygen in the subject gas is measured by means of the electromotive force of the concentration cell.
In this prior art oxygen sensor, the oxygen ion conducting solid electrolyte is sandwiched, for example, between the electrodes, which are formed of a porous, gas-permeable material such as platinum (Pt), and a protective layer of a porous ceramic material is formed on the surface of that one of the electrodes in contact with the subject gas.
An alternative example of the conventional oxygen sensor comprises an insulating supporter, formed of e.g. alumina, and an oxygen concentration detecting element disposed in the supporter. The detecting element is composed of a chip and a pair of Pt electrodes. The chip is disposed in a rectangular recess in one side face of the supporter so that its one side face is exposed. The Pt electrodes, which are connected to the back face of the chip, is used to detect the change of the electric resistance of the chip. The chip is formed of titanium (TiO.sub.2) or some other material which changes its internal electric resistance when it is touched by oxygen, depending on the oxygen concentration.
Generally, however, the conventional oxygen sensors are low in responsiveness. The trouble, therefore, is that once the air-fuel ratio shifts considerably to the lean side due to acceleration of an automobile, for example, the oxygen sensor continues to deliver a lean signal to the electronic control device, even though the air-fuel ratio actually is returned substantially to the stoichiometric value on the rich side. Properly speaking, the air-fuel ratio of the air-fuel mixture supplied to the engine should be adjusted to a value approximate to the stoichiometric value immediately after the end of the acceleration. Due to the response delay of the oxygen sensor, however, a so-called rich excursion occurs such that the ratio deviates to the rich side by a large margin.
In contrast with this, when the air-fuel ratio of the mixture is returned to the value near the stoichiometric air-fuel ratio after it is shifted substantially to the rich side, the oxygen sensor awkwardly continues to deliver a rich signal.
Thus, if the air-fuel ratio of the air-fuel mixture supplied to the engine deviates from the stoichiometric value, the purifying capability of CO, HC, and NO.sub.X of the three-way catalyst decreases sharply, so that the concentrations of these substances in the exhaust gas increase.
These problems have been solved by the development of an improved oxygen sensor which is disclosed in Japanese Patent Disclosure No. 60-256045. In this sensor, the electrodes are formed of an electrically conductive material which is prepared by adding rhodium and at least an oxide of a rare-earth element to platinum. Thus, the catalyst activity of the electrodes is improved, and the response speed of the oxygen sensor is increased.
Disclosed in Japanese Patent Publication No. 57-12002 is an exhaust emission control device which uses an oxygen sensor located on the upper-course side of a three-way catalyst. In this device, a catalytic element having the same catalytic function as the three-way catalyst is disposed in the oxygen sensor itself or on the upper-course side thereof. According to this arrangement, the position in which the oxygen concentration detecting performance of the oxygen sensor, as compared with the air-fuel ratio, suddenly changes can be brought in line with the position in which the optimum purification efficiency of the catalyst can be obtained. Thus, the three-way catalyst can be worked effectively.